Modern telephone systems use tone dialing, and require tone encoders to generate dual frequencies corresponding to one of 16 keys activated from the telephone keyboard. For example, in order to transmit a signal representing a five, the two frequencies, 770 Hz and 1336 Hz are transmitted. Present methods of implementation utilize a bridge-stabilized oscillator employing a T-network bridged by mutual inductance. Two separate tunable inductors are used as a portion of the frequency controlling device. Thus, factory calibration is a requirement. The achievable frequency tolerance is approximately .+-.1.5 percent with initial adjustment, temperature, and aging being the primary controlling factors. Such oscillators also possess the characteristic of frequency pulling. In other words, the row frequency determining elements are not truly independent of the column frequency determining elements.